1. Field of the Invention
The present invention relates to a material for forming a semiconductor device, and more particularly, to a polymer for use in a chemically amplified photoresist used in photolithography, a photoresist composition containing the polymer, and to a method of preparing the polymer.
2. Description of the Related Art
Current semiconductor chips are highly integrated and require complicated manufacturing processes. For example, photolithography processes are needed that are capable of forming fine patterns in the range of 0.25 .mu.m or less. Such fine patterns must be formed using deep ultraviolet (UV) rays of 248 nm, which have a shorter wavelength than conventional g-line (436 nm) and I-line (365 nm) rays. However, when deep-UV rays are used, a smaller number of protons are transferred when a dose of the same energy as a conventional light source is irradiated. Therefore, a dose of much greater energy is required when deep-UV rays are used in order to transfer the same number of protons and obtain the same result as is achieved using conventional photolithography. To overcome this problem, a new material referred to as a "chemically amplified photoresist" has been introduced, which is highly sensitive to protons, due to its improved photosensitivity even when deep-UV rays are irradiated at a similar dose to that used with conventional light source irradiation.
In general, the chemically amplified photoresist includes an acid-labile group which is easily subjected to hydrolysis by an acidic catalyst, and which functions as a dissolution inhibitor. The amplified photoresist also includes a photosensitive acid generator for generating protons H.sup.+ (i.e., acid) by exposure to light. When the chemically amplified photoresist is exposed to light, acid is generated by the photosensitive acid generator. The dissolution inhibitor which is bound to the backbone of the polymer, is then hydrolyzed by the catalytic reaction of the generated acid, thereby changing the polarity (e.g., solubility) of the polymer. Acid hydrolysis of the polymer by the diffusion of acid produces a pattern having a higher transparency.
Thus, contrast, (i.e., an index representing the difference in solubility of a chemically amplified photoresist before and after the exposure to light) is determined by the acid-labile group bound to the backbone of the polymer.
U.S. Pat. No. 4,491,628 discloses a chemically amplified photoresist containing a polymer and using t-butoxycarbonyl (t-BOC) as an acid-labile group. However, this chemically amplified photoresist has a thermal decomposition temperature (Td) that is lower than its glass transition temperature (Tg). Thus, if the photoresist is baked to a temperature that is above the glass transition temperature before exposure to light in order to vaporize unwanted organic solvent and make the photoresist film hard, the photoresist decomposes. On the other hand, if the pre-baking temperature is low enough to prevent thermal decomposition, airborne contaminants can be absorbed into the surface of the exposed photoresist film, thereby preventing a catalytic reaction by the acid. This results in a pattern having an inferior profile, such as T-top profile. T-top profiles are the result of airborne contaminants absorbed into the photoresist film surface which neutralize the acid that is generated by the photoresist upon exposure to light. As a result, some portions of the exposed photoresist do not undergo acid hydrolysis and thus remain insoluble. The insoluble portions of the photoresist are not developed by the developer and result in T-top profiles.